Liquid water, essential for life, exhibits extraordinary physical and biochemical properties largely attributed to its hydrogen bond network. The dielectric susceptibility (χ(ω)), reflecting collective molecular motions in the GHz-THz range, is strongly influenced by intermolecular mode dynamics. Techniques like far-infrared spectroscopy, Raman spectroscopy, and THz time-domain spectroscopy provide insights into hydrogen bond network structure and dynamics, but determining dominant contributions to χ(ω) remains challenging due to complex interactions and spectral overlap. Time-resolved optical Kerr effect (OKE) is a powerful tool, but its application to intermolecular hydrogen bond motions in water is limited by the weak birefringence signal. The time-resolved THz Kerr effect (TKE) offers a solution, as the THz field resonates with rotational transitions and low-frequency molecular motions, enhancing the molecular response. Previous studies using THz pulses on liquids like DMSO and water have tracked molecular dynamics via transient birefringence, focusing primarily on orientational relaxation. However, significant intermolecular modes like hydrogen bond bending (~1.8 THz) and stretching (~5.7 THz) vibrations remain largely unexplored due to high absorption coefficients at these frequencies and the use of cuvettes which complicate data interpretation. This study utilizes an intense, ultrabroadband THz pulse to excite a free-flowing water film, overcoming these limitations and enabling observation of the bipolar TKE signal for the first time, attributed to hydrogen bond dynamics.

Several studies have investigated the dynamics of liquids using THz pulses and transient birefringence. Zalden et al. (2018) used a low-frequency single-cycle THz pulse to stimulate liquid water and observed birefringence due to dipole moment orientation, concluding that the polarizability parallel to the dipole moment is smaller than that perpendicular to it. However, this study was limited by the low power and narrow bandwidth of the THz pulse. Novelli et al. conducted a narrow-band THz pump/THz probe experiment using a free-electron laser, observing a large third-order response at 12.3 THz but lacking time-resolved measurements. These studies primarily focused on orientational relaxation, neglecting the significant intermolecular hydrogen bond vibrations. The use of cuvettes in prior studies also posed challenges in signal interpretation due to the overwhelming response of the cuvette material itself.

The experiment employed an ultrabroadband THz pulse (center frequency 3.9 THz, bandwidth 1–10 THz, peak electric field strength 14.9 MV/cm) to excite a gravity-driven, free-flowing water film. Transient birefringence was monitored using time-delayed optical probe pulses. A schematic of the experimental setup is provided, showing the THz pump pulse and optical probe pulse at a 45° angle to the water film. The resulting bipolar signals, exhibiting clear oscillatory characteristics, were measured under varying THz electric field strengths. Data normalization confirmed the dominance of the Kerr effect. A theoretical model was developed using a hydrogen bond harmonic oscillator model combined with the Lorentz dynamic equation. This model considers the perturbations in the dielectric tensor due to intermolecular vibration modes (bending and stretching). The dielectric susceptibilities parallel and perpendicular to the hydrogen bond direction were expressed as functions of bending and stretching vibration amplitudes (Qb and Qs). The Lorentz dynamic equation describes the motion of the damped harmonic oscillator, with parameters for damping coefficient (γi) and inherent frequency (ωi) taken from the literature. The model was used to simulate the experimental data, decomposing the signals into contributions from electronic effects, Debye relaxation, and hydrogen bond bending and stretching vibrations. Different low-pass filters were used to vary the frequency range of the THz pump pulse, further validating the model. A Fourier transform of the Lorentz equations provided theoretical frequency-domain Kerr coefficients for the bending and stretching modes, which were compared to experimental results. A Kerr coefficient equation (K_eff(ω)) was derived, relating the coefficient to the scaling constants of the THz Kerr effect (ki).

The study observed, for the first time, a bipolar TKE signal in liquid water with significant oscillatory characteristics. The bipolar signals were successfully decomposed into contributions from electronic effects, Debye relaxation, and hydrogen bond bending and stretching vibrations. The positive polarity was attributed to hydrogen bond stretching vibration, and the negative polarity to hydrogen bond bending vibration. The theoretical model, based on a hydrogen bond harmonic oscillator model and the Lorentz dynamic equation, accurately simulated the experimental data, validating the assignment of molecular motions to the observed signals. The model showed that the molecular contribution to birefringence exhibits strong frequency dependence due to the participation of different molecular motion mechanisms. At the high THz excitation frequency used (~3.9 THz), the intermolecular modes dominate the response, unlike lower frequency excitations where orientation processes are more prominent. The frequency-domain Kerr coefficient (K_eff(ω)) was extracted from experimental results, showing a peak near 4.5 THz corresponding to the contribution of hydrogen bond stretching vibration. The measured Kerr coefficient could be well-approximated by the sum of the two intermolecular mode contributions. The values of scaling constants k1 and k2 were estimated from experimental data.

The findings provide a more intuitive time-resolved evolution of polarizability anisotropy induced by the dynamics of collective intermolecular modes in liquid water on a sub-picosecond scale. The successful decomposition of the bipolar TKE signal into contributions from hydrogen bond stretching and bending vibrations provides direct evidence for the role of these modes in determining the ultrafast dynamics of liquid water. The developed theoretical model accurately captures the experimental observations, enhancing our understanding of the interplay between intermolecular interactions and the resulting optical properties. The frequency dependence of the molecular response highlights the importance of considering the specific excitation frequency when investigating liquid water dynamics. The results challenge previous interpretations that solely relied on orientational relaxation and underscore the significant contribution of hydrogen bond vibrations.

This study demonstrates the effectiveness of using an intense, ultrabroadband THz pulse to investigate the ultrafast intermolecular hydrogen bond dynamics of liquid water. The observation of a bipolar TKE signal and its successful decomposition into stretching and bending vibrational contributions provide novel insights into water's transient structure. The developed theoretical model provides a framework for interpreting future experiments in this area. Future research could explore the effects of temperature and pressure on these dynamics or investigate other hydrogen-bonded liquids.

The study focuses on liquid water under ambient conditions. Extending the investigation to different temperatures and pressures could provide a more comprehensive understanding of the hydrogen bond dynamics. The model simplifies the complex interactions within the hydrogen bond network. Incorporating more detailed descriptions of intermolecular interactions could improve the accuracy of simulations.